This disclosure relates to metal tubing and methods of manufacture for same.
The metal tubing of this disclosure may be used in coiled tubing strings. Coiled tubing is a continuous length of spooled metal tubing. A metal tubing can in practice be formed from a flat composite metal strip that has been made from two individual strips welded end-to-end. The composite metal strip is then rolled into a cylindrical/tubular shape and seam-welded at the longitudinal edges to form a length of tubing. Coiled tubing can be produced in various lengths depending on specific application requirements, up to or exceeding 10,000 meters (or about 30,000 feet). Because of the coiled tubing's significant length, coiled tubing is commonly stored or transported on a reel.
During operation, coiled tubing is repeatedly spooled onto and unspooled from its reel. Coiled tubing is also under variable internal pressurizations, including super- and sub-atmospheric loads that result in various tension-compression stress and strain states. Additional loads to the coiled tubing can include bending loads (e.g., when spooled), axial loads (e.g., when supporting own gravity), and torsional loads (e.g., when twisted). Different stress and strain states can exist on different sections of coiled tubing. Variations in the stress and strain states can limit the coiled tubing's total life. For example, when coiled tubing is spooled out of a vertical well, the section at the reel is under bending loads, while the section inside the well is under axial loads. The cyclic loading of these various stress and strain states upon spooling and unspooling can result in a limited life due to fatigue.
The metal tubing's total life is measured by the number of spooling/unspooling cycles. Industry has established common coiled tubing life based on empirical data and theoretical predictions. Yet coiled tubing may still occasionally and prematurely fail when the cyclic loading causes cracks to initiate and/or propagate in the tubing. The cracks are usually located at or near a strip-to-strip weld, due to mechanical property differences between the weld and the strip's metal material. For example, fracture failures happen more often at strip-to-strip weld.